Next-generation non-volatile memory devices with lower power consumption and higher degree of integration than flash memory devices are being studied. Such next-generation non-volatile memory devices include phase-change random access memory (PRAM) that uses state changes of a phase change material such as chalcogenide alloys, magnetic random access memory (MRAM) that uses resistance changes in a magnetic tunnel junction (MTJ) depending on the magnetization state of a ferromagnetic material, ferroelectric random access memory (FRAM) that uses polarization of a ferroelectric material, resistance-change random access memory (ReRAM) that uses resistance changes in a variable resistance material, and the like.
Examples of MRAM include a spin-transfer torque magnetic random access memory (STT-MRAM) device that inverts magnetization using a spin-transfer torque (STT) phenomenon generated by electron injection and discriminates a resistance difference before and after magnetization inversion. The STT-MRAM device includes a magnetic tunnel junction, which consists of a pinned layer and a free layer, each formed of a ferromagnetic material, and a tunnel barrier disposed therebetween. In the magnetic tunnel junction, when the magnetization directions of the free layer and the pinned layer are the same (that is, parallel), current flow is easy and consequently the magnetic tunnel junction is in a low resistance state. On the other hand, when the magnetization directions are different (that is, antiparallel), current is reduced and consequently the magnetic tunnel junction is in a high resistance state. In addition, in the magnetic tunnel junction, the magnetization directions must change only in the direction perpendicular to a substrate. Therefore, the free layer and the pinned layer must have perpendicular magnetization values. When the perpendicular magnetization values are symmetrical with respect to 0 according to the intensity and direction of a magnetic field, and a squareness (S) shape becomes clear (S=1), perpendicular magnetic anisotropy (PMA) is considered to be excellent. The STT-MRAM device is theoretically capable of cycling more than 1015 times and can be switched at a high speed of about a few nanoseconds (ns). In particular, a perpendicular magnetization type STT-MRAM device is advantageous in that there is no theoretical scaling limit, and as scaling progresses, the current density of driving current may be lowered. Therefore, the perpendicular magnetization type STT-MRAM device has been actively studied as a next-generation memory device that may replace DRAM devices. An example of the STT-MRAM device is disclosed in Korean Patent No. 10-1040163.
In the STT-MRAM device, a seed layer is formed on the lower part of the free layer, a capping layer is formed on the upper part of the pinned layer, and synthetic antiferromagnetic layers and an upper electrode are formed on the upper part of the capping layer. In addition, in the STT-MRAM device, a silicon oxide film is formed on a silicon substrate, and then the seed layer and a magnetic tunnel junction are formed thereon. In addition, a selection element such as a transistor may be formed on the silicon substrate, and the silicon oxide film may be formed so as to cover the selection element. Therefore, the STT-MRAM device has a laminated structure in which a silicon oxide film, a seed layer, a free layer, a tunnel barrier, a pinned layer, a capping layer, synthetic antiferromagnetic layers, and an upper electrode are formed on a silicon substrate on which a selection element is formed. In this case, the seed layer and the capping layer are formed using tantalum (Ta), and the synthetic antiferromagnetic layers have a structure in which a lower magnetic layer and an upper magnetic layer, in which a magnetic metal and a non-magnetic metal are alternately stacked, are formed, and a non-magnetic layer is formed therebetween.
However, since the seed layer formed on the upper part of the amorphous silicon oxide film is amorphous and the magnetic tunnel junction is also amorphous, the crystallinity of the magnetic tunnel junction may be deteriorated. That is, the pinned layer and the free layer are formed of amorphous CoFeB. In this case, the crystallinity of the magnetic tunnel junction is not greatly improved even when heat treatment is performed to improve perpendicular anisotropy characteristics. When the crystallinity of the magnetic tunnel junction is low, perpendicular magnetic anisotropy may be lowered. Therefore, even when a magnetic field is applied to change the magnetization direction, the magnetization direction does not change rapidly, and the amount of current flowing in a parallel state is reduced. As a result, read/write time is delayed, which makes it difficult to implement a high-speed memory device, and operation errors may occur during read/write operation.
In addition, a metal line forming process and a passivation process are performed after formation of the synthetic antiferromagnetic layers and the upper electrode. At this time, these processes are performed at a temperature of about 400° C. However, when Ta is used as the seed layer, the perpendicular magnetic anisotropy of the magnetic tunnel junction is lowered at a temperature of about 400° C. Therefore, it is necessary to improve the thermal stability of the perpendicular magnetic anisotropy of the magnetic tunnel junction.